Stable lead alkyl compositions and a method for preparing the same



United States Patent ()fiiee 2,992,250 Patented July 11, 1961 2,992 250 STABLE LEAD ALKYLOMPOS'IT IONS AND A METHOD FOR PREPARING THE SAME 'Hymin Shapiro, Baton Rouge, La., and Herbert RJNeaH,

This invention relates to alkyllead compositions which are stable at temperatures above 100 C. It also relates to methods for inhibiting the thermal decomposition of alkyllead compounds when subjected to temperatures above ,100" C., at which temperature thermal.decompsition becomes appreciable.

Generally this invention contemplates inhibiting the thermal decomposition of alkyllead compounds in which atleast one valence of the lead is satisfied by an alkyl radical.

More specifically this invention is concerned with an improved process for separating alkyllead compounds from the reaction products accompanying their synthesis. It is also applicable to a method for inhibiting thermal decomposition of an alkyllead product during its purification and blending with other products in making commercial antiknock fluids. It is applicable to minimizing the possibility of thermal decomposition during storage or transportation of an alkyllead product. It is especially applicable to preventing thermal-decomposition of undiluted alkyllead compounds where the likelihood of thermal decomposition is more ofya problem.

As is well known, tetraalkyllead antiknock compounds generally are produced by reacting a sodium-lead alloy with an alkyl halide. Due to recent marked improvements in the technology of alkyllead manufacture, thermal instability of alkyllead compounds during synthesis is no longer a problem. However, the tetraalkyllead compound so produced is in admixture with various reaction byproducts from which it must be separated. Separation is effected by steam or vacuum distillation with subsequent purification of the tetraalkyllead distillate. Due to the toxic and unstable nature of tetraalkyllead ,antiknock compounds, these distillation andvpurification operations are subject to many difficulties.

In these distillation and purification operations, meticulous temperature control and exact safetymeasures are of paramount importance. The rate of decomposition of the alkyllead compound increases rapidly with small rises in temperature above .the temperature where thermaldecomposition becomes appreciable. For example, decomposition of tetraethyllead occurs atthe rate of approximately 2 percent-per-hour at a'temperature of 100 C., which is the customary temperature used in separating tetraethyllead jfrom -the reaction products accompanying itssynthesis. Attemperaturesabove 100 C. the decomposition rate increases logarithmically so that a point is soon reached Where external heat is no longer required and decomposition becomes self-propagating.

Such likelihood of excessive decomposition is present also during blending, handling, storage, and transportation. Prior to diluting the alkyllead concentrate with scavengers, gasoline or other materials, the alkyllead compound remains a concentrate and the problem of excessive thermal decomposition exists, even though the temperature is maintained normally well below that of decomposition. For example, in the purification step wherein the tetraethyllead concentrate is washed and blown with air at atmospheric temperature to remove impurities, a sudden increase in temperature may occur due to the oxidation of triethylbismuth which is present as an impurity. Also pumps used in handling tetraethyllead Occasionally freeze and the friction developed may cause a local overheating to a temperature above the temperature of decomposition of the tetraethyllead. Faulty wiring, leaks-onto steam pipes, and other accidental causes also may produce local overheating with resulting dangerous decomposition.

It'is seen therefore that inthose operations where an alkyllead compound is in the undiluted or .concentrated state-viz. separation, purification, blending, transportation, and storage-the likelihood of excessive thermal decomposition must be provided for and efiectively combatted.

An object of this invention is to stabilize alkyllead compounds against thermal decomposition during one or more of the'following operations: separation, purification, blending, transportation, and storage.

This object is accomplished by incorporating with alkyllead compounds a relatively small quantity of a material which has the property of inhibiting alkyllead thermal decomposition. This object is also accomplished by conducting one or more of the foregoing operations in the presence of such a'material. The materials which have been foundto possess the property are referred to hereinafter as thermal stabilizers.

The thermal stabilizers of this invention are dialkyl benzenes where the total number of alkyl carbon atoms is at least 4. These thermal stabilizers when used in amounts varyingfrom about 0.5 to about 10 percent by weight of the lead alkyl product are effective in substantially retarding or preventing thermal decomposition at temperatures above C. for extendedperiods of time.

Another important feature of this invention is that the foregoing'thermal stabilizers areinexpensive and abundant. A further advantage in using them is that they are not corrosive to metals used in fabricating alkyllead storagetanks, pipe lines, tank cars, storage drums, and the like.

Preferred because of their elegant solubility characteristics are the above defined dialkyl benzenes which are liquid at ordinary temperatures. These liquid dialkyl benzenes rapidly and completely dissolve in the alkyllead compound at ambient temperatures. Once so dissolved, there is no tendency for subsequent formation and precipitation of crystals or other undesirable sediments even at low temperatures.

Typical thermal stabilizers of this invention are: 1,4- di-tert-butyl benzene; 1-methyl-3-isoamyl benzene; l-ethyl- 2-hexyl benzene; 1,4-dioctyl benzene; 1,3-di-(1,1,2-tnmethylpropyl) benzene; 1,4-di-(1,1,3,3-tetramethylbntyl) benzene; l-ethyl-3-decy1 benzene; 1,2-diodecyl benzene; and, in general, dialkyl benzenes where the total number of alkyl carbon atoms ranges from 4 to about 24, each alkyl group preferably containing up to about 12 carbon atoms. Examples of the preferred liquid thermal stabilizers of'this'invention include: 1,2-diethyl benzene; 1,3- diethyl benzene; IA-diethyl benzene; 1-ethyl-4#isobutyl benzene; 1-ethyl-3-isopropyl benzene; 1-ethyl-4-isopropyl benzene; l-ethyl-4-propyl benzene; 2-isopropyl-l-methyl benzene; 3-isopropyl-1-methyl benzene; 4-isopropyl-1- methyl benzene; 1-butyl-2-methyl benzene; 1-butyl-3- benzene; 1-butyl-4-methyl benzene; l-methyl-Z-propyl benzene; l-methyl-3-propyl benzene; l-methyl-4-propyl benzene, and the like.

The chief thermal decomposition products of alkyllead compounds are lead metal and hydrocarbon gas. Hence, a very good index of alkyllead thermal decomposition is liberation of this gas.

To illustrate the effectiveness of this invention, direct comparisons were made between the decomposition characteristics of unstabilized and stabilized tetraethyllead. A thermostatically controlled hot oil bath was fitted with a stirrer, thermometer, and a holder for a small reaction tube. A 100 cc. gas buret beside the bath, and equipped with a water-containing levelling bottle, was connected by means of rubber tubing with the reaction tubeafter the desired sample was introduced into this tube. After the bath was brought to a steady, temperature, of 160f C., the sample-containing tube was quickly immersed in the bath and clamped with the levelling bottle adjusted to hold the gas buret in place at -a zero reading. Then measured was the time during which the samplewas held at 160 C. without pronounced thermal decomposition and consequent gas evolution occurring. Thus, the longer the time, the more thermally stable was the alkyllead composition.

With pure tetraethyllead used in 1 ml. amounts, pronounced thermal deterioration occurred within 1 minute as evidenced by rapid gas evolution. In fact, the decomposition of unstabilized tetraethyllead will normally become uncontrollable if it is'heated, whether rapidly or slowly, to even 130 C., unless it is possible to very rapidly cool it down to about 100 C. or less.

However, when to the same amount of tetraethyllead there was previously added 2 percent by weight of diethyl benzene no pronounced thermal deterioration occurred at 160 C. for over 30 minutes. Likewise, when this experiment was repeated using 2 percent by weight of cymene (methyl isopropyl benzene) no pronounced thermal deterioration occurred for over 20 minutes. The same order of effectiveness subsists when repeating these experiments using in one instance 2 percent by Weight of 1-butyl-3-methyl benzene and in another instance 4 percent by weight of 1,3-di-(1,1,3,3-tetramethylbutyl) benzene. In fact, comparable elfectiveness is exhibited by all of the thermal stabilizers of this invention.

An important facet of this invention is the discovery that the thermal stabilizers must contain a benzene ring substituted with 2 alkyl groups having a total of at least 4 carbon atoms. Departure from these important structural criteria markedly impairs the elfectiveness of the compound as a thermal stabilizer. By way of example, 2 percent by weight of 1,3-diphenyl benzene (a typical diaryl substituted benzene) and of xylene (a dialkyl benzene having a total of less than 4 carbon atoms in the alkyl groups) both failed to produce any detectable inhibition of the tetraethyllead thermal decomposition at 160 C. when tested in the manner described above.

The above-dmcribed beneficial behavior of the thermal stabilizers of this invention also takes place with other alkyllead compounds such as triethyllead bromide and tetrapropyllead. In fact, these compounds when stabilized can be boiled and distilled at atmospheric pressure.

This invention is adapted to the stabilization of tetraethyllead and other alkyllead compounds at various stages after they have been formed and the diluents or excess alkyl halide have been discharged from the autoclave. For example, one of the above thermal stabilizers may be added in appropriate quantity to the alkyllead reaction concentrate just before the separation step which is conducted at a temperature close to the temperature where hazardous run-away decomposition is particularly prevalent. By adding one of the above thermal stabilizers to the reaction concentrate just prior to distillation, the danger arising from unexpected temperature increases is substantially eliminated.

Most preferably the above thermal stabilizers are employed to stabilize the alkyllead com-pound both in storage and in shipping and especially to stabilize any alkyllead concentrate, i.e., compositions containing at least percent by weight of alkyllead compound. If elevated temperature conditions are likely to be encountered, the addition of a small amount of thermal stabilizer to the alkyllead compound will economically and satisfactorily eliminate most of the hazard involved. While meticulous temperature control and exacting safety measures have been successful in reducing to a minimum the hazards of processing and handling of tetraethyllead, the use of this invention provides a much greater factor of safety. Furthermore, waste of the alkyllead product due to decomposition is considerably minimized through the use of this invention.

This invention is useful in stabilizing alkyllead compounds in which at least one valence of the lead is satisfied by an alkyl radical. For example tetraethyllead, tetramethyllead, tetrapropyllead, dimethyldiethyllead, triethylphenyllead, and triethyllead bromide can be successfully stabilized against thermal decomposition by incorporating therein a relatively small quantity of one of the thermal stabilizers of this invention.

What is claimed is:

1. A method of inhibiting the decomposiition of an alkyllead compound at temperatures from about C. to about C. which comprises incorporating with said compound a small amount of a dialkyl benzene where the total number of alkyl carbon atoms is from 4 to about 24 sufficient to inhibit said decomposition.

2. The method of claim 1 wherein said dialkyl benzene is a liquid.

3. In the process of producing an alkyllead compound by reacting sodium lead alloy with alkyl chloride and separating the thus-produced alkyllead compound from the reaction mass by steam distillation, the step which comprises conducting said steam distillation in the presence of a small amount of a dialkyl benzene where the total number of alkyl carbon atoms is from 4 to about 24 sufficient to inhibit thermal decomposition of the alkyllead compound.

4. An alkyllead compound containing, in amount sufficient to inhibit thermal deterioration thereof at temperatures from about 100 C. to about 160 C., a dialkyl benzene where the total number of alkyl carbon atoms is from 4 to about 24.

5. The composition of claim 4 wherein said dialkyl benzene is a liquid.

Calingaert Nov. 24, 1953 Krohn Dec. 13, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,992,250

' July 11, 1961 Hymin Shapiro et a1.

It is hereby certified that err ent requiring correction and that th or appears in the above numbered patcorreoted below.

e said Letters Patent should read as Column 2, lines 59 and 60, for "lbutyl-S-benzene" read lbutyl-3methyl benzene Signed and sealed this 5th day of March 1963.

(SEAL) Attest:

ESTON G. JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents 

1. A METHOD OF INHIBITING THE DECOMPOSITION OF AN ALKYLLEAD COMPOUND AT TEMPERATURES FROM ABOUT 100*C. TO ABOUT 160*C. WHICH COMPRISES INCORPORATING WITH SAID COMPOUND A SMALL AMOUNT OF A DIALKYL BENZENE WHERE THE TOTAL NUMBER OF ALKYL CARBON ATOMS IS FROM 4 TO ABOUT 24 SUFFICIENT TO INHIBIT SAID DECOMPOSITION. 